Apparatus for varying the path length of a beam of radiation

ABSTRACT

An apparatus for varying the path length of a beam of radiation, the apparatus comprising: an element ( 51 ) rotatably mounted about an axis, said element comprising two reflective surfaces in fixed relation to one another such that radiation may be reflected between said reflective surfaces and out of the element ( 51 ); and driving means ( 55 ) for rotatably oscillating said element about said axis.

The present invention relates to the field of optics. More specifically,the present invention relates to a scanning delay line for investigativesystems and particularly those that operate in the frequency rangecolloquially referred to as the TeraHertz frequency range. Thisfrequency range being the range from 25 GHz to 100 THz, particularly therange from 50 GHz to 84 THz, more particularly the range from 90 GHz to50 THz and especially the range from 100 GHz to 20 THz.

Scanning delay lines are used when it is necessary to sweep the phase ofone beam of radiation with respect to another. For example, in THzimaging systems such as those described in GB 2 347 835, two beams ofradiation are used: a first irradiating beam, which travels from anemitter to a detector via a sample; and a second, reference beam. Inorder to obtain information about the sample the phase of one of the twobeams needs to be continually scanned relative to the other beam. Thisscanning step is achieved by providing a scanning delay line in the pathof one of the beams which continuously varies the path length of one ofthe beams. Scanning delay lines are also used in Optical CoherenceTomography (OCT), where it is again necessary to vary the phase of onebeam of radiation in a Michelson interferometer arrangement with respectto another.

Currently, there are two main types of scanning delay lines. The firstof these is provided by retroreflectors which turn the beam through 180°and which are capable of linear reciprocating motion, thus extending orshortening the optical path as required.

The faster the operation of the scanning delay line, the faster theacquisition of information about the sample being investigated. It isnot possible to move a retroreflector at high speeds required forapplications such as imaging and thus, these types of delay lines are oflimited use.

The second main type of scanning delay lines is provided by providing aconstantly rotating optic within the path of one of the beams ofradiation. The radiation reflects off at least one surface within theoptic and thus the optical path changes as this surface rotates. Thesetypes of delay lines tend to work quicker than retroreflectors, but theysuffer from the problem that they do not produce a linear variation inthe phase over time and suffer from a very low duty cycle. Also, themaximum and minimum delay introduced by the scanning delay line may notbe changed once with delay line has been assembled.

Modifications on the above design include work by Delachenal et al,Optics Communications 162 195-199 (1999) who use a delay line comprisinga rotating polygonal mirror and a fixed glass cube and Ballif et al,Optics Letters 22, 757-759 (1997) who use a delay line comprising arotating cube which allows internal reflection and a fixed prism.

Further attempts to solve the above problems have generally been made inrelation to OCT imaging systems. Tearney et al, Optics Letters 21, 1408to 1410 (1996) use a piezo-electric fiber stretcher to continually varythe length of the optical path of one radiation beam with respect to theother. This solution to the problems of the prior art is unlikely to beof much use in THz systems which often use pulses of radiation as suchpulses will be dispersed in the optical fibers.

Rollins et al, Optics Express, 3 219-239 (1998) use a scanning delayline which comprises a single mirror and which receives light from anddirects light back onto a diffraction grating. Scanning is achieved bytitling the mirror under the control of a resonant scanner.

Finally, Riffe and Sabbah, Review of Scientific instruments, 69,3099-3101 explain how a rotating prism affects the delay length of ascanning delay line in order to determine the maximum possible pathlength as a function of the dimensions of the prism. The prism used is arhomboid prism and it is driven by a stepper motor.

It is an object of the present invention to at least address some of theproblems of the prior art and produce a high speed scanning delay line.

In a first aspect, the present invention provides an apparatus forvarying the path length of a beam of radiation, the apparatuscomprising: an element rotatably mounted about an axis, said elementcomprising two reflective surfaces in fixed relation to one another suchthat radiation may be reflected between said reflective surfaces and outof the element; and driving means configured to rotatably oscillatingsaid element about said axis.

When placed in the path of a beam of radiation, the element reflectsincident radiation as it moves with respect to the beam. At certainrotation angles, the element will fail to correctly reflect the incidentradiation. Radiation which is correctly reflected enters the element andis reflected by a first mirror onto a second mirror and possiblysubsequent mirrors, the radiation is finally directed out of the elementalong a suitable radiation path which allows the beam to be directed bya further mirror or collected by an element such as a detector.Radiation which is incorrectly reflected will either fail to bereflected by the first mirror or will be reflected by the first mirrorat an angle which will prevent it from being reflected by one or more ofthe subsequent mirrors, or it will be reflected by the mirrors withinthe element but will not exit the element along a suitable radiationpath such that it will not be collected or re-directed by other elementsin the optical circuit.

By oscillating the element as opposed to fully rotating the element, itis possible to minimise the angles, which the control element scansthrough, which result in incident radiation being incorrectly reflected.Thus the duty cycle of the element may be increased to values in excessof 90%. Preferably, the driving means is configured to oscillate saidelement through an angle of at most 40°.

The driving means preferably comprises a galvanometer. A Galvanometercomprises a coil located in a magnetic field. If a current is passedthrough the coil, the coil experiences a torque proportional to thecurrent passed through the coil. Therefore, the rotation imparted by agalvanometer allows the position of the element to be finely controlled.

The galvanometer may be configured such that either rotation of the coilwith respect to a fixed magnet, which generates the magnetic field, orrotation of the magnet with respect to a fixed coil is used to positionthe element.

Also, since the torque applied to the element via the galvanometer isdependent on the current applied to the galvanometer, the speed of theelement can be varied throughout each oscillation thus allowing theelement to vary the delay length, linearly with respect to time.

The element may be operated to scan linearly at a frequency of about 100Hz. If the element is scanned sinusoidally, frequencies of 300 Hz to 400Hz may be achieved.

Since the element is oscillated, the start and end points of theoscillation may be controlled by the driving means. Thus, the minimumand maximum points of the oscillation may be controlled by the drivingmeans and hence, the minimum and maximum points of the scanning delayline are set by the driving means and may be altered by the drivingmeans.

The element may be provided by two or more separate mirrors which arefixed with respect to one another. However, preferably, the elementcomprises a solid optic and said reflective surfaces are provided bysurfaces of said optic. More preferably, said solid optic is a prism andpreferably a rhomboid prism where said surfaces are two mutuallyparallel surfaces of said rhomboid.

If a solid optic is used, the apparatus may be operated such thatinternal reflection conditions are satisfied. Said reflective surfacesare preferably metallised.

Preferably, the material of the solid optic has a higher refractiveindex than its surroundings. For example, typically, solid optic will bemainly surrounded by air and thus a refractive index of greater than 1is advantageous. Preferably at least 1.1, more preferably at least 1.2,even more preferably at least 1.5. By using a material with a higherrefractive index, the range of angles over which radiation can becorrectly reflected is increased thus allowing a large delay to beintroduced by the element. Using an element with a refractive index of1.7 allows the variation in the path length introduced by the element tobe doubled over that of an element with a refractive index of 1 forcertain geometries. Although the overall variation in the delay isincreased, the element may still be driven to linearly vary the delay byusing a galvanometer. The glass SF11 has a high refractive index.

As the element oscillates back and forth, the path of radiation exitingthe element differs dependent on the rotation angle of the element. Thisvariation in the exiting path is undesirable since radiation exiting theelement will generally be collected or deflected by further opticalelements. Therefore, it is preferable to keep the exiting path the sameregardless of the rotation angle of the element. This may be achieved byproviding a reflecting member configured to reflect radiation exitingthe element back into the element, such that radiation reflected backinto the element exits the element along a fixed final exit pathregardless of the rotational position of the element.

Radiation which enters the element for a first time follows a firstpath, the reflecting member may be configured to reflect radiation backinto the element such that the radiation reflected by the reflectingmember follows the first path in reverse. Preferably, in order to beable to distinguish between radiation reflected back through saidelement and radiation which is to enter said element, the reflectingmember is provided with polarisation translation means which allows thepolarisation of the reflected radiation to be different to that enteringthe element for the first time. For example, the polarisation may berotated by 90° or changed from being circularly polarised in onedirection to being circularly polarised in the other direction. Thisallows the radiation to be separated by using a polarising beam splitteror the like.

Alternatively, the reflecting member may be configured to reflectradiation back into the element such that the reflected radiationfollows a second path, said second path being said first path reversedand displaced along said rotation axis.

Preferably said reflecting member is a first reflecting member and theapparatus further comprises a second reflecting member, said first andsecond reflecting members being configured such that radiation may bereflected back through said element at least four times. In aparticularly preferred arrangement radiation is passed through theelement 6 times. The reflecting elements may be configured to reflectthe radiation along parallel paths within the element. By reflectingradiation back through the element four or more times, the variation inthe delay introduced by the element can be increased. Although theoverall variation in the delay is increased, the element may still bedriven to linearly vary the delay by using a galvonometer.

In a second aspect, the present invention provides a method for varyingthe path length of a beam of radiation, the method comprising:

-   -   providing an element comprising two reflective surfaces in fixed        relation to one another such that radiation may be reflected        between said reflective surfaces and out of the element;    -   rotatably mounting said element about an axis; and    -   rotatably oscillating said element about said axis.

The present invention may be used in any type of investigative orimaging system where a scanning delay line is required. For example, itmay be used in OCT systems or Terahertz imaging systems or systems whereanalysis or identification of a sample is required.

In a third aspect, the present invention provides a system forinvestigating a sample, the system comprising:

-   -   an emitter for emitting radiation to irradiate said sample;    -   a detector for detecting radiation reflected from or transmitted        by said sample, radiation travelling from the emitter to the        detector following a first path;    -   means for supplying radiation along a second path to said        detector and having a phase related to that of the radiation        leaving the emitter,    -   the system further comprising an apparatus according to the        first aspect of the invention, provided within either of the        first or second paths.

In the system of the third aspect of the invention, the emitterpreferably emits radiation in the THz frequency range. The system may bean imaging system or any other type of investigative system, for examplea spectroscopic system.

The present invention will now be described with reference to thefollowing non-limited preferred embodiments in which:

FIG. 1A schematically illustrates an apparatus in accordance with afirst embodiment of the present invention in a first position and FIG.1B schematically illustrates the apparatus of FIG. 1A in a secondposition;

FIG. 2 is a schematic which demonstrates how the path length ofradiation passing through the apparatus of FIG. 1A varies with therotation angle of the apparatus of FIG. 1A;

FIG. 3A is a plot of the path length in millimetres against the rotationangle θ of the apparatus of FIG. 1A, FIG. 3B is a plot of displacementalong the Y-axis of the maximum height of mirror M1, the minimum heightof mirror M1 and the height at which the incident radiation impinges onmirror M1 against rotation angle, and FIG. 3C is a plot of displacementalong the Y-axis in millimetres of the maximum height of mirror M2, theminimum height of mirror M2 and the height of the radiation reflectedfrom mirror M2 against the rotation angle;

FIG. 4 is a schematic of an apparatus in accordance with a furtherembodiment of the present invention;

FIG. 5 is a schematic of an apparatus in accordance with a furtherembodiment of the present invention;

FIG. 6 is a schematic of the radiation paths through the prism of FIG.5;

FIGS. 7 a to 7 c are schematics illustrating how the angle of rotationthrough which radiation is correctly reflected may be enhanced byincreasing the refractive index of the material used to form theelement; and

FIG. 8 is a schematic of an investigative system in accordance with afurther embodiment of the present invention.

FIG. 1A illustrates an apparatus in accordance with a first embodimentof the present invention in a first position. The apparatus comprises anelement 1 which is rotatable about a central axis 3 by a rotation means5. The rotation means 5 are only schematically shown in this diagram andwill not further described in relation to this figure.

Element 1 is a rhomboid prism having two pairs of parallel planarsurfaces which may be rotated through the path of a radiation beam. Afirst pair of parallel surfaces 7, 9 are used for entry and exit of theradiation beam from the prism 1, second pair of parallel surfaces 11, 13are used to reflect the beam while travelling through the prism.Reflection may occur either due to the angle of the incident beamresulting in total internal reflection conditions being meet and/orsurfaces 11, 13 may be metallised in order to enhance reflection.

In the example of FIG. 1A, radiation enters at the lowest part ofsurface 7 and is reflected from surface 11 onto reflective surface 13.Reflective surface 13 then reflects the light out through exit surface9.

FIG. 1B illustrates the same prism as FIG. 1A but at a differentrotation angle. To avoid unnecessary repetition, like reference numeralswill be used to denote like features.

Due to the rotation of element 1, reflective surface 11, 13 are in adifferent positions in FIG. 1B to FIG. 1A.

In FIG. 1B, the different position of reflective surfaces 11, 13 causesthe beam to be essentially reflected backwards whereas in FIG. 1A, thebeam still travels in a generally forward direction but is deflectedslightly by mirrors 11, 13. Hence, by comparing FIGS. 1A and 1B, it canbe seen that the radiation path length from point Ps to point PB differsas element 1 is rotated about axis 3.

How the rotation affects the path length can be seen more clearly fromthe graph of FIG. 2 and the results of FIG. 3.

In FIG. 2, reflective surf aces 13, 11 are renumbered as mirrors M1 andM2 respectively. The central axis A of the mirrors is rotated by angle θabout the Y-axis. Angle φ indicates the angle of the two mirrors M1 andM2 with respect to central axis A. θ represents the rotation angle ofthe element about the rotation axis which coincides with the origin. Thediagram also illustrates the distance R which is the distance along thecentral axis A from the rotation axis to the centre of mirror M1 ormirror M2.

Radiation 21 enters the element at height H along the Y-axis andimpinges on mirror M1. It is then reflected along path 23 to mirror M2where it leaves the element along path 25. The positions of paths 23 and25 will vary dependent on the rotation angle θ of the mirrors about therotation axis, parameters h, r and φ will remain the same regardless ofrotation angle θ.

FIG. 3A, 3B and 3C illustrate results where φ=45°, r=7 mm, h=9 mm andthe length of the mirrors is 10 mm.

FIG. 3A illustrates displacement along the Y-axis in millimetres againstθ along the X-axis in degrees. The displacement corresponds to theincrease or decrease of the path. When θ=0, i.e. the central axis Aaligns with the Y-axis, the length of the path which the radiation takesalong paths 21, 23 and 25 is 14 mm. As the element 1 is rotatedanti-clockwise, this path length increases due to the radiation beingreflected back on itself. Approximately 5 mm is added to the path lengthwhen θ is turned through 30° anti-clockwise. Similarly as element 1 isrotated clockwise, the path length decreases and at 30° the path lengthis found to decrease to about 5 mm.

FIG. 3B illustrates three plots. The central fixed line at 9 mmcorresponds to the height h where the radiation first impinges on mirrorM1. The upper most curve corresponds to the maximum height of thesurface of mirror M1 above the X axis. The lower most curve correspondsto the minimum height of mirror M1 from the X axis.

In order for radiation entering the prism to impinge on first mirroredsurface M1, it is important that the height of the incident radiation isbetween the maximum mirror height and the minimum mirror height. It canbe seen that this situation is not true for θ is excess of 17.5°. Here,the maximum height of mirror M1 falls below the height of the incidentradiation, therefore, the incident radiation will just simply bypass theprism and would not be reflected.

FIG. 3C illustrates information about second mirror M2. The trace whichis in the middle when θ=−30° corresponds to the maximum height of themirror measured below the X axis of FIG. 2 and the lowest trace at θ=30°corresponds to the minimum height of mirror M2 below the X axis. Theupper most trace of θ=30° corresponds to the distance d along the Y-axiswhich corresponds to the distance d from the X axis of FIG. 2 at whichthe beam leaves the element along path 25. In order for the beam to bereflected from mirror M2, this height must fall between the maximum andminimum heights of mirror M2.

It can be seen from FIG. 3C that this condition is only satisfied forthe small range of angles between θ=−7.5° and θ=20°. Therefore,comparing the results from FIGS. 3B and 3C, it can be seen that theprism is only effective between the angles θ=−7.5° and θ=17.5° for thespecific dimensions given above. Please note, that these figures maychange dependent on the angle of mirrors M1 and M2 to central axis A andthe radius r of mirrors M1 andM2.

Thus, to obtain maximum efficiency from the element configured as above,it is only necessary to rotate the element through 30°.

Changing the length of the mirrors will also affect the above graphs asthe length of the mirrors will affect the maximum and minimum heights ofthe mirrors plotted in FIGS. 3A and 3B. Extending the mirrors willdecrease the minimum heights and increase the maximum heights and istherefore initially desirable. However, if the mirror lengths areincreased, the top mirror M1 of FIG. 2 may occlude the lower mirror M2thus preventing the incident beam from entering the element.

FIG. 4 illustrates an apparatus in accordance with a further embodimentof the present invention.

Rhomboid prism 51 is provided to reflect radiation. Rhomboid prism 51 ismounted upon rotational control arm 53 which is in turn controlled bygalvanometer 55. By energising galvanometer 55, the control arm can berotated back and forth such that prism 51 oscillates back and forthabout its rotation axis.

Galvanometer 55 comprises a coil located in a magnetic field. If acurrent is passed through the coil, the coil experiences a torqueproportional to the current passed through the coil. Therefore, therotation imparted by a galvanometer allows the position of the elementto be finely controlled.

Also, since the torque applied to the element via the galvanometer isdependent on the current applied to the galvanometer, the speed of theelement can be varied throughout each oscillation thus allowing theelement to vary the delay length linearly with respect to time.

Radiation enters the device through radiation path 57 and is reflectedthrough prism 51 following a first radiation path and out along eitherexit path 59 or 61 depending on the position of the prism 51. In thefigure, exit path 59 illustrates the position of the exit beam when theprism is positioned such that the beam experiences the maximum delay andexit path 61 illustrates the position of the exit beam when the prism ifpositioned such that the beam experiences the minimum delay.

When the beam experiences the maximum delay and exits via exit path 59,it impinges on roof prism 63 which is a right angled prism. Roof prism63 reflects radiation beam 59 through 180° and along path 65 which isparallel to exit path 59 and displaced from path 59 in the direction ofthe rotation axis of the prism 51.

Radiation 65 which is reflected from roof prism 63 enters prism 51 andis internally reflected by the surfaces identified in relation to FIGS.1A, 1B and 2 following a second radiation path, and exits the prism 51along radiation path 67.

Similarly, when the beam experiences the minimum delay and exits viaexit path 61, it impinges on roof prism 63 and is reflected through 180°and along path 69. Exit path 69 is parallel to exit path 61 anddisplaced from path 61 in the direction of the rotation axis of theelement 51.

Radiation 69 which is reflected from roof prism 63 enters prism 51 andis internally reflected by the surfaces identified in relation to FIGS.1A, 1B and 2 following a second radiation path and exits the prism 51along radiation path 67.

Regardless of the rotation position of the prism 51, the secondradiation path is the first radiation path reversed and displaced alongthe rotation axis. Thus, radiation always exits the prism along finalexit path 67 which is parallel to path 57 and at a fixed displacementtherefrom. Hence, the scanning delay line may be easily incorporatedinto any optical system since its beam exit path remains fixedregardless of the delay introduced by the oscillating prism 51.

Although not shown here, in use, the galvanometer 55, control arm 53 andprism 51 will be provided on a stepper motor to adjust prism 51 to thecorrect position for the system.

FIG. 5 schematically illustrates a variation on the apparatus of FIG. 4.To avoid unnecessary repetition, like reference numerals will be used todenote like features.

FIG. 5 primarily differs from FIG. 4 in that the beams are configured topass through prism 51 six times.

Radiation enters the prism 51 through radiation path 101 and isreflected through prism 51 following a first radiation path and outalong exit path 103. The vertical position of exit path 103 variesdepending on the rotation angle of prism 51. In the figure, exit path103 illustrates the position of the exit beam when the prism 51 ispositioned such that the beam experiences the maximum delay.

When the beam experiences the maximum delay and exits via exit-path 103,it impinges on large roof prism 105 which is a right angled prism. Roofprism 105 reflects radiation beam 103 through 180° and along path 107which is parallel to exit path 103 and displaced from path 103 in thedirection of the rotation axis of the prism 51.

If the prism 51 is oriented such that the beam experiences the minimumdelay, it exits the prism 51 at a position which is vertically shifteddownwards from path 103 (not shown). The beam is still reflected through180° by large roof prism 105 and the beam exits roof prism 105 alongpath 109 which is vertically shifted downwards from path 107.

Radiation following path 107 or 109 from roof prism 105 enters prism 51and is internally reflected by the surfaces identified in relation toFIGS. 1A, 1B and 2 following a second radiation path, and exits theprism 51 along radiation path 111.

Regardless of the rotation position of the prism 51, the secondradiation path is the first radiation path reversed and displaced alongthe rotation axis. Thus, radiation always exits the prism along exitpath 111 which is parallel to path 103 and at a fixed displacementtherefrom.

Radiation exiting prism along path 111 then impinges on lower roof prism113 and is reflected through 180°. The beam exits lower roof prism 113along path 115 which is parallel to and horizontally shifted from path111.

Radiation following path 115 is then reflected back through rotatingprism 51 along a third path which is parallel to the first and secondpaths. Radiation then exits the prism 51 along path 117. Path 117 isreflected through 180° by upper roof prism 105 such that the radiationexits roof prism 105 along path 118. Path 117 is the exit path forradiation experiencing the maximum delay due to prism 51.

Radiation following path 118 is then reflected back through rotatingprism 51 along a fourth path which is parallel to the first, second andthird paths. Radiation then exits the prism 51 along path 119. Path 119is reflected through 180° by lower roof prism 113 such that theradiation exits roof prism 113 along path 121. Path 119 is the exit pathfor radiation regardless of the rotation position of the prism 51.

Radiation following path 121 is then reflected back through rotatingprism 51 along a fifth path which is parallel to the first, second,third and fourth paths. Radiation then exits the prism 51 along path123. Path 117 is reflected through 180° by upper roof prism 105 suchthat the radiation exits roof prism 105 along path 125. Path 123 is theexit path for radiation experiencing the maximum delay due to prism 51.

Radiation following path 125 is then reflected back through rotatingprism 51 along a sixth and final path which is parallel to the first tofifth paths. Radiation then exits the prism 51 along path 127. Path 127is the exit path for radiation regardless of the rotation position ofthe prism 51. Hence, the scanning delay line may be easily incorporatedinto any optical system since its beam exit path remains fixedregardless of the delay introduced by the oscillating prism 51.

FIG. 6 is a schematic of the first to sixth radiation paths throughprism 51. To avoid unnecessary repetition, like reference numerals willbe used to denote like features.

The first and second radiation paths (paths 1 and 2) are seen to be atthe furthest ends of prism 51. The third and fourth radiation paths(paths 3 and 4) are the innermost paths. The fifth path (path 5) isbetween the second and fourth paths and the sixth path (path 6) isbetween the first and third paths. This arrangement is chosen becausethe roof prisms are each symmetric. The lower roof prism 113 is providedoffset from the upper prism 105 in order to achieve the described pathsthrough the prism 51.

By multiply reflecting the radiation back through the prism 51, thedelay introduced by the prism may be increased.

FIGS. 7 a to 7 c schematically illustrate the effect of using a solidoptic as the prism and fabricating the solid optic from a material witha high refractive index. FIG. 7 a schematically illustrates the useableangle of through which the prism 151 maybe rotated when the refractiveindex inside the prism matches the refractive index of the environmentof the prism. This angle is seen to be 16.08° for the geometry of theprism of FIG. 7 a. The useable angle is the angle through which theprism may be rotated and still correctly reflects and directs theradiation. A description of correct and incorrect reflection is givenwith respect to FIGS. 1 and 2.

FIG. 7 b schematically illustrates the useable angle of through whichthe prism 151 may be rotated when the refractive index is of the prismis slightly higher than the refractive index of the environment of theprism. The useable angle in this case for the same geometry of FIG. 7 ais 22.80°.

FIG. 7 c schematically illustrates the useable angle of through whichthe prism 151 may be rotated when the refractive index is of the prismis slightly higher than the refractive index of the environment of theprism. The useable angle in this case for the same geometry of FIG. 7 ais 36.80°.

Thus, by increasing the refractive index of the material of the solidoptic, the prism may be rotated through a larger angle which stillallows correct reflection of the radiation. Thus, the path lengthvariation introduced by the prism may be substantially enhanced byincreasing the refractive index of the prism.

The delay element described with reference to FIGS. 1 to 7 is primarilyintended for use in investigative systems and primarily those whichoperate using THz radiation. Such a system is schematically shown inFIG. 8.

Laser 201 outputs a beam of radiation 203, this is divided by beamsplitter 205 into a pump beam 207 and a probe beam 209. The radiationemitted by laser 201 will typically be in the mid-infrared range. Inorder to produce THz radiation, the mid-infrared pump beam 207 impingeson THZ emitter 211. THz emitter 211 comprises a photoconductivesubstrate and two electrodes provided on said substrate. Terahertz beam213 is emitted from emitter 211 and is guided via mirrors 215 ontosample 217. Sample 217 is provided on a motorised stage which allows itto move in the X and Y directions. The X and Y directions beingorthogonal to one another such that a large area of the sample may bescanned. The reflected radiation is then directed via mirrors 219 ontodetector 221.

In this type of imaging system, a probe beam 209 is also provided to thedetectors 221. In order to determine the phase shifts introduced bysample 217, the phase of probe beam 209 is varied over a small range inorder to continually vary the phase of the probe beam reaching detector221. To achieve this, the pulse beam is passes through scanning delayline 223 which has been described with reference to FIGS. 1 to 7.

1. An apparatus for varying the path length of a beam of radiation, theapparatus comprising: an element rotatably mounted about an axis, saidelement comprising two reflective surfaces in fixed relation to oneanother such that radiation may be reflected between said reflectivesurfaces and out of the element; and driving means for rotatablyoscillating said element about said axis.
 2. An apparatus according toclaim 1, wherein said driving means comprises a galvanometer.
 3. Anapparatus according to claim 2, wherein said driving means is configuredto vary the speed of the element during each oscillation such that thepath length is varied linearly with time.
 4. An apparatus according toclaim 1, wherein said driving means is configured to oscillate saidelement through an angle of at most 40°.
 5. An apparatus according toclaim 1, wherein said element comprises a solid optic and saidreflective surfaces are provided by surfaces of said optic.
 6. Anapparatus according to claim 5, wherein the said reflective surfaces aremetallised.
 7. An apparatus according to claim 5, wherein said solidoptic is a rhomboid prism and said surfaces are two facing surfaces ofsaid rhomboid.
 8. An apparatus according to claim 5, wherein the solidoptic comprises a material having a higher refractive index than
 1. 9.An apparatus according to claim 8, wherein the solid optic has arefractive index of at least 1.2.
 10. An apparatus according to claim 1,further comprising a reflecting member configured to reflect radiationexiting the element back into the element, the reflecting member beingconfigured such that radiation reflected back into the element exits theelement along a fixed final exit path regardless of the rotationalposition of the element.
 11. An apparatus according to claim 10, whereinradiation which enters the element for a first time follows a first pathand the reflecting member is configured to reflect radiation back intothe element such that the radiation reflected by the reflecting memberfollows the first path in reverse.
 12. An apparatus according to claim11, wherein said reflecting member is provided with polarisationtranslation means.
 13. An apparatus according to claim 10, whereinradiation which enters the element for a first time follows a first pathand the reflecting member is configured to reflect radiation back intothe element such that the reflected radiation follows a second path,said second path being said first path reversed and displaced along saidrotation axis.
 14. An apparatus according to claim 10, wherein saidreflecting member is a first reflecting member and the apparatus furthercomprises a second reflecting member, said first and second reflectingmembers being configured such that radiation may be reflected backthrough said element at least four times.
 15. A method for varying thepath length of a beam of radiation, the method comprising: providing anelement comprising two reflective surfaces in fixed relation to oneanother such that radiation may be reflected between said reflectivesurfaces and out of the element; rotatably mounting said element aboutan axis; and rotatably oscillating said element about said axis.
 16. Asystem for investigating a sample, the system comprising: an emitter foremitting radiation to irradiate said sample; a detector for detectingradiation reflected from or transmitted by said sample, radiationtravelling from the emitter to the detector following a first path;means for supplying radiation along a second path to said detector andhaving a phase related to that of the radiation leaving the emitter, thesystem further comprising an apparatus according to claim 1, providedwithin either of the first or second paths. 17-19. (canceled)